November 21, 2013

Revisiting Lunar Soil Data From Apollo Program

In 1969, Neil Armstrong took man's first otherworldly steps onto the lunar surface. He had no idea what a nuisance the soil beneath his feet would prove to be at the time. Scratchy lunar dust clung to everything it touched. This caused scientific instruments to overheat and a type of lunar dust hay fever for Apollo 17 Astronaut Harrison Schmitt. NASA undertook a scientific experiment to figure out how fast the dust collects, but the data was lost - or so NASA thought.

Forty years later, the data has been recovered and scientists have used it to make the first determination of how fast lunar dust accumulates. The dust builds up unbelievably slow by Earth standards, according to the study - just fast enough to form a layer about a millimeter (0.04 inches) thick every 1,000 years. That is still 10 times faster than previous estimates. It is also fast enough to pose serious challenges for the solar cells that serve as critical power sources for space exploration missions.

“You wouldn’t see it; it’s very thin indeed,” said University of Western Australia Professor Brian O’Brien, a physicist who developed the experiment while working on the Apollo missions in the 1960s and now has led the new analysis. “But, as the Apollo astronauts learned, you can have a devil of a time overcoming even a small amount of dust.”

O'Brien added that the faster-than-expected pile-up also implies that the lunar dust could have more ways to move around than scientists previously believed.

Dust collected on small solar cells attached to a matchbox-sized case over the course of six years, throughout three Apollo missions. The voltage that the solar cells produced dropped off as the dust granules blocked the light coming in. Each year, 100 micrograms of lunar dust collected per square centimeter, according to the electrical measurements. At such a rate, a basketball court on the Moon would accumulate approximately 450 grams, or 1 pound) of lunar dust annually.

O'Brien compared the effects on cells of the dust and of damaging high-energy radiation from the sun. He found that long-term dust accretion could diminish the output from shielded power supplies of a lunar outpost more than even the most intense solar outbursts.

Solar cell manufacturers fortify their devices against radiation damage because the threat it poses was recognized early on. However, “while solar cells have become hardier to radiation, nothing really has been done to make them more resistant to dust,” said O’Brien’s colleague on the project Monique Hollick, who is also a researcher at the University of Western Australia, in Crawley. “That’s going to be a problem for future lunar missions.”

NASA scientists realized the Lunar Module would likely kick up a large amount of lunar soil on takeoff before Apollo 11 launched its mission to the Moon in 1969. The researchers realized that this would potentially coat nearby scientific experiments with dust. Detachable covers were investigated, but they would require either a small explosive or a physical mechanism to remove after the astronauts left, which would create more engineering challenges and room for failure.

“Then I asked what I thought was a pretty common sense question,” recalled O’Brien. “If we’ve got to guard ourselves against damage from the lunar module taking off, who’s measuring whether any damage actually took place; who’s measuring the dust?”

As a small add-on device to larger experiments, O'Brien invented the Lunar Dust Detector experiment. The dust detector, which required little power and weighed only 270 grams (0.6 pounds), reported back to Earth alongside the non-scientific housekeeping data.

“It really got a free ride,” O’Brien said.

The small detectors flew on Apollo missions 12, 14 and 15. They remained active until NASA shut them down in September 1977 due to budgetary concerns. NASA did not preserve the archival tapes of the data they collected while the detectors worked properly. NASA assumed the data was lost forever until 2006 when O'Brien told them he still had a set of backup copies.

For the experiment, each detector had three solar cells. Each of these cells was covered with a different amount of shielding against incoming radiation. O'Brien compared the damage to the unshielded and shielded solar cells, finding that dust, rather than radiation, caused the most degradation to the protected cells.

Before this experiment, model-based estimates of lunar dust accumulation assumed the dust came entirely from meteor impacts and falling cosmic dust. “But that’s not enough to account for what we measured,” O’Brien said.

The Moon's soil should be stagnant without an atmosphere for wind. O'Brien said, however, that a popular idea of a “dust atmosphere” on the Moon could explain the difference. According to the concept, during each lunar day, solar radiation is strong enough to knock a few electrons out of atoms in dust particles, which builds up a slight positive charge. A small negative charge is created on the nighttime side of the Moon as electrons from the flow of energetic particles, called the solar wind, which comes off the Sun and strikes dust particles. On the border between the light and dark regions of the surface, electric forces could levitate this charged dust, potentially lofting grains high into the lunar sky.

“Something similar was reported by Apollo astronauts orbiting the Moon who looked out and saw dust glowing on the horizon,” said Hollick.

NASA's Lunar Atmosphere and Dust Environment Explorer (LADEE) launched in September, which could soon confirm the idea of levitating lunar dust. LADEE will orbit 155 miles above the surface of the Moon searching for dust in the lunar atmosphere.

O'Brien reflects on a decades-long science experiment that finally has results as LADEE begins to scour the Moon's atmosphere.

“It’s been a long haul,” said O’Brien. “I invented [the detector] in 1966, long before Monique was even born. At the age of 79, I’m working with a 23-year old working on 46-year-old data and we discovered something exciting—it’s delightful.”

The results of this study were published in a recent issue of Space Weather.